Any material, regardless whether it is homogeneous or composite, can be processed in any number of different ways. And, depending on the desired results, the procedure that is selected for processing the material can be performed using operational parameters that may need to be changed during the procedure. Of particular interest here are laser systems that process or alter material for purposes of cutting, reshaping or removing portions of the material(s). As is well known, laser processes for doing this typically involve phenomena such as laser induced optical breakdown (LIOB), photodecomposition, or photoablation.
In recent years (i.e. since the invention of the laser in the 1960s) laser systems have been effectively used to alter or process a significant number of different type materials. More recently, it has been recognized that laser beams which have laser pulses of ultra short duration (e.g. picosecond and femtosecond duration) are particularly effective for many applications. Normally, such laser systems are operated at a fixed level of pulse energy, with a fixed pulse repetition rate. Thus, it has been a standard practice to determine the energy level that is required in laser pulses to effectively process a target material(s). A pulse repetition rate that will maintain this energy level is then accepted. If lower energies would be needed of sections of a processing procedure, the output energy of the laser would be simply reduced using well known types of attenuators while maintaining the same pulse repetition rate. This, however, does not consider the fact that changes in a pulse repetition will result in changes in the energy level of the laser pulses in the beam. It happens for many applications that this fact may be advantageously used.
Referring to FIG. 1, the relation of pulse energy and repetition rate in a typical ultra short pulse laser beam is shown. Specifically, FIG. 1 shows that as the repetition rate (R) of pulses in a laser beam is increased, the energy level (E) of the pulses decreases. Stated differently, the energy level in each pulse is dependent on the pulse repetition rate, and they vary inversely. As indicated above, this trend may be used to advantage because many, in fact most, materials are not homogeneous. Thus, such materials (e.g. composites) will have different energy thresholds for ablation, and therefore require different energy levels to alter or process different sections of the material. Furthermore, even in homogeneous materials, different sections within a processing procedure may require different energy levels.
In light of the above, it is an object of the present invention to provide a system and a method for predetermining the energy level required to alter or process a section of material, and then using the corresponding maximum pulse repetition rate with a view toward reducing the time required to perform a material processing procedure. Another object of the present invention is to provide a system and method for processing a material that effectively employs a variable pulse repetition rate to minimize the time required to perform a material processing procedure. Still another object of the present invention is to provide a system and a method for processing a material that selectively varies the pulse repetition rate of a laser beam, either pursuant to pre-programmed instructions or in response to closed loop feedback controls. Yet another object of the present invention is to provide a system and a method for processing a material that is relatively simple to manufacture, is easy to use, and is comparatively cost effective.